Insulation on older house
i know two different contractors whom i both respect, and posed this same question to both of them, regarding vapor barrier.
one of them said to use the poly, the other one said not to.
i think if you asked 100 builders, it would be split right down the middle.
here's my own opinion:
fiberglass sucks in general and will not get you air-tightness. i will never use it unless i have no other choice. i would hesitate to use poly, especially in a bathroom where you'll have lots of steam. i foresee all kinds of condensation issues.
if it were my house i would, without any hesitation, spend the extra money on expanding foam insulation. it might not even cost any more.
i recently did a bathroom addition, was able to have the whole thing sprayed, floor and ceiling full, and walls, for under $500. i could not have installed fiberglass for any less than that (labor and materials).
if you insist on using fiberglass, my own recommendation would be to get the faced batts, installing the facing toward the heated space, without the poly barrier. i don't have a very technical reason for this, but that's what i would do.